1. Field of the Invention
The present invention relates to a dielectric film deposition method in the fabrication of semiconductor devices, and more particularly, to a method for forming a silicon nitride film in a plasma enhanced chemical vapor deposition (PECVD) batch type chamber.
2. Description of Related Art
Generally, PECVD equipment deposits a material formed by a chemical reaction in gas through electric discharge on a semiconductor substrate (i.e., a wafer) to form a dielectric film. In PECVD batch type equipment, deposition of layers or films of material is repeatedly performed on wafer batches. When a predetermined amount of film is deposited in a PECVD batch type equipment chamber, RF plasma cleaning is performed to remove film that has built up in the chamber. However, a temperature inside the chamber lowers after the RF plasma cleaning process, causing a decrease in deposition rate. This phenomenon occurs each time the RF plasma cleaning process is performed.
As a consequence of the decrease in deposition rate, the thickness of film deposited on the wafers is low before a first RF plasma cleaning process is performed and becomes abruptly high immediately after a second RF plasma cleaning process is performed. Therefore, due to differences in the deposition rates before and after the RF plasma cleaning process, thickness of films deposited on wafers may differ depending on when the batch of wafers underwent the deposition process (e.g., thick if immediately after an RF plasma cleaning process and thin if immediately before).
FIG. 1 is a flowchart illustrating a process of depositing silicon nitride films on wafers using a conventional PECVD batch type equipment.
Referring to FIG. 1, before silicon nitride film deposition occurs, pre-coating is performed by depositing a silicon oxide/nitride film in a chamber at step S100. Such silicon oxide/nitride film pre-coating prevents particle sources from being generated during the RF plasma cleaning and prevents the first wafer effect. The first wafer effect results in a different deposition rate for the first batch of wafers as compared to subsequent batches. For example, without silicon oxide/nitride film pre-coating, the inside of a chamber is not covered with a nitride film in a process for a first sheet or batch of wafers, but it is covered with a nitride film in a process for the next batch of wafers. This difference in conditions may cause a difference in deposition rates for each batch of wafers due to a difference of deposition rates on the inner surface of the chamber.
After pre-coating (s100), a batch of wafers on which a nitride film is to be deposited in a PECVD method is inserted into the chamber, and a PECVD silicon nitride film deposition process is performed at step S102. The deposition for each batch of wafers is performed for the same amount of time.
During the silicon nitride film deposition step, the silicon nitride film is formed on not only the wafers but also the inner surface of the chamber, a heater, and a shower head. Therefore, after the silicon nitride film deposition is performed on a predetermined number of wafers, the entry of a batch of wafers into the chamber is blocked, and an RF plasma cleaning process is performed at step S104. In the RF cleaning process, all the wafers are removed out of the chamber, and a cleaning gas such as HF, SiF4, or Ar is injected to remove the silicon nitride film formed on the inner surface of the chamber, heater, and shower head. After the RF plasma cleaning process, a pump/purge process in the chamber is repeated several times to remove the remaining gas in a gas line and to remove impurities such as particles generated during the cleaning process, at step S106.
As described above, according to the process of depositing silicon nitride films on wafers of the related art, the thickness of the silicon nitride film is low before a first RF plasma cleaning process is performed and becomes abruptly high immediately after a second RF plasma cleaning process is performed. As this phenomenon occurs each time the RF plasma cleaning process is performed, the silicon nitride films differ in thickness from one batch of wafers to another due to the difference in the deposition rates before and after the RF plasma cleaning process. This effect is shown in FIG. 2 which shows a graph illustrating the thickness of the nitride films gradually decreasing after an RF plasma cleaning period.
The variation in film thickness may also cause a variation in capacitance values in metal insulator metal (MIM) nitride applications and a variation in Vt resistance values in pre-metal dielectric (PMD) linear film applications. These undesirable variations contribute to a reduction in overall manufacturing yield.